1. Field of the Invention
This invention generally relates to methods for forming a light emitting device, and more particularly to a technique for lifting-off a sapphire substrate from a light-emitting diode.
2. Description of the Related Art
Semiconductor light-emitting devices have developed rapidly in many applications, for example, general lighting and liquid crystal display backlighting. As such, semiconductor light-emitting devices may replace currently used illumination devices, such as fluorescent lamps or light bulbs. Specifically, GaN based light-emitting diodes represent a main focus in such application developments. FIG. 1 shows the structure of a typical conventional light emitting device, wherein an n-GaN layer 104, quantum wells 106, and a p-GaN layer 108 are sequentially formed on a sapphire substrate 102, and a p-type electrode 110 connects to the p-GaN layer 108, and an n-type electrode 112 connects to the n-GaN layer 104. With the sapphire substrate, which is electrically and thermally insulating, such a light emitting device, however, has the disadvantage of long current path, such that the device resistance is high. Also, the thermally insulating nature of sapphire substrate makes it difficult in removing the generated heat.
Accordingly, a vertical light-emitting diode (VLED) has been developed. As shown in FIG. 2, a vertical light-emitting diode is formed by placing a p-i-n structure (including an n-type GaN layer 204, quantum wells 206 and an p-type GaN layer 208) and a sapphire substrate 202 thereunder upside down, and bonding a p-type electrode 210 (connecting the p-type GaN layer 208) with an electrically and thermally conducting substitution substrate 212. Next, the sapphire substrate 202 is lifted off to expose the n-type GaN layer 204, and the n-type GaN layer 204 is surface-polished for forming an n-type electrode 214 thereon for the fabrication of a light emitting diode with vertically flowing current. Compared to the conventional horizontal light emitting diode, the vertical light emitting diode has the advantages of being able to remove heat more easily and has better current spreading effect. Therefore, a vertical light emitting diode is more suitable to be used as a high power light emitting device. When fabricating a vertical light emitting diode, a substrate lifting-off process is the key technological component. Currently, an ultraviolet laser is used to illuminate the sapphire substrate and scan the light emitting diode structure to lift off the sapphire substrate. Since the material between the sapphire substrate and gallium nitride has a high defect density and strong light absorption, heat generated from laser irradiation can separate the sapphire substrate from the gallium nitride. Although this technique is widely used, it has some drawbacks. For example, laser irradiation may damage the material of the quantum wells in the light emitting diode. Also, semiconductor chips are likely to be broken. Therefore, it is important to develop a novel sapphire substrate lift-off technique for high power light emitting diodes.